Rick's Free Auto Repair Advice

Store Lawn mower with or without gas

Should you store your lawnmower with or without gas?

If you do an online search to find out whether you should store your lawnmower with or without gas you’ll find two contradictory answers. One approach, usually pitched by companies that sell fuel stabilizers tells you to fill your tank to the brim with freshly stabilizer fuel. The other, pitched by some small engine manufacturers, tells you to run the gas tank dry and then continue pulling the starter cord to run all gas out of the carburetor so it’s bone dry. Which approach is right? Read on to find the pros and cons of each approach

What happens to gas when it’s left in a small engine in the offseason?

If the fuel contains ethanol, the alcohol will separate out of the gas after about 30-days. That’s 30-days from when you got it from the gas station, not 30-days from when you filled the tank on your small engine.

Since alcohol is hygroscopic, it adsorbs water and, since water is heavier than gas, the alcohol/water blend falls to the bottom of the tank. The water enters the gas tank from moisture in the surrounding air that works its way through the open vent in the gas cap. The process of alcohol “separating” from the gas  is called “phase separation.” The alcohol/water combination will corrode the steel tank and the metals in the carburetor. Chances are very high that next spring the engine won’t start.

In addition to phase separation, old gasoline degrades in several ways. First, since gasoline is a blend of up to 150 different components, the most volatile components evaporate, leaving behind a thicker and thicker mixture that’s harder to ignite when you go to start the engine next season. Next, the remaining components oxidize, leaving a coating of varnish and gum inside the tank and clogging the small passages of the carburetor.

corroded lawnmower carburetor

What does fuel stabilizer do?

First, fuel stabilizer prevents or slows down phase separation, in some cases for up to 24-months (although I’ve never believed that). Next, fuel stabilizers contain metal de-activators that stop chemical reactions caused by dissolved metals in fuel. In other words, it prevents metal corrosion. Stabilizers also contain detergent ingredients that prevent gum and varnish build-up on engine parts. Finally, the best stabilizers also contain a seal conditioner to prevent swelling in rubber gaskets, O-rings, and the neoprene or Viton needle and seat.

Back to the question—should you leave the lawnmower tank full or empty?

Pros to running the engine and carburetor dry

• You don’t have to worry about phase separation, gum and varnish buildup or metal corrosion
• You don’t have to buy fuel stabilizer

Cons of running the carburetor dry

• Even though the tank and carburetor are empty, the carburetor throttle and gas tank are still open to the atmosphere and any moisture in the air can result in corrosion in the tank and carburetor passages. This is the con that most stabilizer manufacturers use to justify using their products.  There is a simple way to prevent this kind of corrosion though.

Place a baggie over the fuel tank filler and then screw on the cap. That will seal the tank. Then remove the air filter and place a baggie over the air filter holder and secure it in place with a rubber band. That will seal the carburetor intake.

• Leaving fuel in the carburetor can cause the needle and seat to stick, causing a start/stall condition next season. When the carburetor bowl is full, the float exerts maximum pressure on the needle and seat; the “valve” that allows gas to flow into the bowl from the tank. After sitting in the closed position for a full season, the needle can often stick in the fully closed position. When that happens, your small engine will start and then stall once the fuel in the bowl is used up. To fix the stuck needle valve, you sometimes have to open the carburetor to physically unstick the valve.

Pros to filling your lawnmower with stabilized gas

When small engine manufacturers recommend filling the tank fully with stabilized gas, they want you to run the machine for a while first. That causes the stabilized gas to enter the carburetor. So you fill with stabilized fuel, run the engine, stop the engine and then top off the tank again.

Cons to filling the tank with stabilized gas

•  Most people don’t follow the directions regarding the proper mixing of fuel stabilizer with gas (see below).
• Most people don’t run the engine long enough to get stabilized gas into the carburetor.

How to mix fuel stabilizer with gas

Stabilizer MUST be mixed with fresh fuel right at the pump. That means adding it to your gas can before filling the gas can with gas. This allows a complete mix while filling the gas can.

Adding fuel stabilizer after filling the gas can will result in an uneven mix.

Adding fuel stabilizer to old or stale gas is totally ineffective. The stabilizer will NOT bring stale gas back from the dead. It will not reverse any phase separation that has already taken place.

Rick’s advice on whether to store your lawnmower with or without gas

Newer engines use different metals in the carburetors and those metals are more resistant to corrosion. If your lawnmower is newer or has a plastic carburetor and plastic gas tank, store it empty.

If you have an older lawnmower with a metal tank, fill the tank with fresh stabilized gas and seal the tank with a baggie under the gas cap.

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat

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